1. Field of the Invention
The present invention relates to the surface modification of sialon ceramics and surface modified sialon ceramics prepared thereby, wherein the phase of the surface layer in sialon ceramics has the different phase from the bulk phases.
2. Description of Prior Art
Si.sub.3 N.sub.4 has two different structural modifications, .alpha. and .beta.. The solid solution in .beta.-Si.sub.3 N.sub.4, formed by simultaneous equivalent substitution of Al--O for Si--N called .beta.-sialon and the solid solution in .alpha.-Si.sub.3 N.sub.4 called a-sialon.
The general mechanical and thermal properties of sialon ceramics are summarized at Table. 1.
TABLE 1 ______________________________________ The General property comparison of .alpha.- and .beta.-sialon ceramics Properties .alpha.-sialon .beta.-sialon ______________________________________ Hardness (Hv, kg/mm.sup.2) .about.2000 .about.1500 Strength (MPa) 500.about.900400 Fracture Toughness (MPam.sup.1/2 ) 3.about.5 4.about.10 Thermal Oxidation Resistance good ______________________________________
The general formula of .beta.-sialon is, Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z where z varies 0 to 4.2 within solubility range.
The general formula of .alpha.-sialon is M.sub.p Si.sub.12-(m+n) A.sub.(m+n) O.sub.n N.sub.16-n, where p, m and n are variable numbers. M denotes metal ion where, Li, Mg, Ca, Y, Er, Nd, Gd, Dy, Na, Ce, Sm, and Be. In a case of M=Y, Y-.alpha.-sialon(yttrium stabilized alpha sialon), m value varies 1.0 to 2.4 and n varies zero to 1.7 in relation to p-m/3.
In order to fabricate sialon ceramics, additives are usually used such as AlN, Al.sub.2 O.sub.3, Y.sub.2 O.sub.3, MgO, CaO, Li.sub.2 O.sub.3, Sm.sub.2 O.sub.3, Er.sub.2 O.sub.3, Yb.sub.2 O.sub.3, Dy.sub.2 O.sub.3, Gd.sub.2 O.sub.3, wherein positive metal ion penetrate into a network structure of Si.sub.3 N.sub.4 for forming .alpha.-sialon.
Sialon ceramics can be obtained from the powder mixtures of AlN, Al.sub.2 O.sub.3, Y.sub.2 O.sub.3, Si.sub.3 N.sub.4, and the amounts of each powder determine the phase ratio of the .alpha.- and .beta.-phase.
In general, the microstructure of .alpha.-sialon ceramics shows the equiaxed grains with high hardness, better wear and oxidation resistance, while .beta.-sialon ceramics by their elongated grains with better flexural strength and fracture toughness. To combine advantages in physical properties of both sialons, .beta.-.beta. sialon composite has long been studied. For example, comparing with pure .beta.-sialon, .alpha.-.beta. sialon composite shows improved hardness and oxidation resistance with reduction of flexural strength and fracture toughness. These properties vary almost linearly with the formation of added sialon.
The surface modification is another technique to improve the properties of ceramics. The surface modification provides opportunities of introducing surface compressive stresses which result from the difference in thermal expansion coefficient between a surface layer and bulk. Some previous results showed that the surface nitridation of dense SiC could improve surface-sensitive mechanical properties because of the thermally induced compressive stresses at the surface of sialon ceramics.